In conventional methods used for the purpose of shaping and curing thermosetting resins, either with or without electrically conductive materials, partial or complete shaping and curing is carried out by applying heat externally to the composition to be processed. External heat may be applied by the use of electrically, steam, oil or gas heated elements or platens and the material to be cured is confined in a press or die which is also used to shape or compact the material to the required dimensions.
In some methods at least a part of the heat may be generated by transferring the material from a central source, by the application of an external force, via channels connected to various cavities of final dimensions. The friction between the material and the transfer channel walls generates additional heat, thus speeding up the curing reaction.
These methods are generally not altogether satisfactory. Uniform heating of the composition is difficult because of the fact that the resins exhibit poor heat conducting characteristics.
In contrast to the above methods, wherein heat is applied externally, the present invention is directed to the generation of heat within the material itself by a method which ensures uniform heating.
To this end, the present invention provides a method of shaping a composition comprising at least one material including at least one electrically conductive material, dispersed in a substantially non-electrically conductive plastic material selected from thermosetting resins, thermoplastic resins and elastomers; which method comprises (a) introducing the composition into a die or like shaping device, and (b) passing an electrical current through the composition within the shaping device to thereby effect resistance heating and shaping of the composition. It is to be appreciated that the electrical current may be AC or DC and may be direct or induced.
The passage of an electrical current through the composition results in resistance heating which enables shaping (and in some instances curing) to be carried out.
Resistance heating results in the raising of the temperature of the composition at a uniform rate throughout in contrast to conventional method where uniformity is restricted by thermal conductivity. On a microscopic scale, the points of heat generation within the material occur at the most advantageous locations, i.e. at the interfaces between the conductive and non-conductive materials. As a result of heating uniformly throughout the material, the process enables the time scale to be reduced to such a point that it is possible, for a brief time, to reach higher temperatures than can be applied in conventional methods which depend on heat conduction. This allows achievement of a high level of cure without the time being sufficient to cause excessive decomposition and damage to the end properties. Another important factor is that in some cases post-curing, a more or less compulsory step with some products in prior art methods, may be dispensed with.
Conveniently, the composition to be shaped is confined and optionally compressed in the cavity of a die made of electrically insulating material or materials. The die may be heated or cooled depending upon each material formulation and its end use.
Conveniently, the current can be applied to the composition by means of electrodes. These are preferably placed so as to give uniform heating through the composition. However, any pair or multiples of pairs may be connected to different power sources having different potential and current values, the values and the actual connecting network being specific for each shape and/or formulation and the end product requirements.
The actual form of the electrodes is not essential to the invention but is determined by the shape of the end product. The electrodes may take the form of two electrically conductive plates placed on opposite sides of the material. They can also take the form of segment elements placed around the material to be cured.
The actual voltage or current required to heat and cure the composition will naturally be determined by the resistance of the composition itself and the end use requirements.
Depending on the final properties required, and to assist in the achievement of a sound product, the current may be applied in interrupted or uninterrupted fashion. Also, the current may be varied or pulsed throughout the curing cycle, ranging for example from a few amperes to 500 amperes.
The control of the process can conveniently be achieved by monitoring the energy input in terms of watt-seconds thereby making the process independent of variations that occur in resistance from batch or batch or during the curing cycle itself.
For some material compositions or products, it is advantageous to have the conductive electrodes controlled within a specified temperature range. This temperature range will be specific for each material composition or formula. One temperature range found to be particularly satisfactory, however, is from 0.degree. C. to 500.degree. C. The heating of the electrodes facilitates flow of the material into cavities, promotes effective bonding in products where the composition is to be bonded with a previously applied adhesive to a metal backing, minimises moisture condensation and also assists in clean separation of the composition from the electrodes.
Whether or not compression is necessary will depend on the composition itself, the shape and the end use of the finished product. Pressure applied to the composition may vary from no pressure at all to several tons per square inch.
The compositions cured and shaped by the process of the present invention have a variety of end uses. For instance, they may be used in the manufacture of friction materials for use in brakes, clutches and the like, and they may also be used in the manufacture of anti-friction materials for use in solid bearings and machinery slide plates, or of moulded thermosetting articles such as knife handles, plastic gears and household equipment handles. Of particular interest is the area of friction materials manufacture.